Uniaxial minerals

The optical character of chalcedony is distinct from that expected for the normally uniaxial mineral, quartz, and signals the fibrous nature of a particular sample. The direction of fiber elongation is often parallel to the [1120] crystallographic direction of the quartz structure (Fig. 2.19A), but other fiber directions have also been determined within a single sample (Frondel, 1985). The presence of helically twisted fibers are suspected from the variations in extinction and birefringence noted along the fiber length (Fig. 2.19C). More detailed information on the optical or other physical and chemical properties of quartz and its many varieties can be found in volume 3 of Palache et al. (1962) and in Frondel (1985). 

The method of determining n will be described later. If a uniaxial mineral, i.e., a mineral having a unique c axis and omega or epsilon as n is examined, the crystal can have one of three orientations. 

Examination of the four possibilities in biaxial minerals indicates that there is no possibility of seeing the same index in all particles of the mineral in some position of rotation of the stage. A uniaxial mineral will always show omega in some position of rotation. 

The ability to dr aw approximately 100 - A diameter fibrils from PTFE dispersion particles, either by fracture after cold compaction [34] or by uniaxial or biaxial expansion of sheets or tubes after paste extrusion (extrusion of mixtures of the dispersion particles and lubricants such as mineral spirits) and lubricant removal [35], was demonstrated a number of years ago. The former process results in the development of a myriad of fibrils spanning the gap between the fracture faces these were utilized for ED characterization of the PTFE conformation and crystal packing. 

If optical examination shows two n s, the two indices will be omega and epsilon or e1, and the mineral belongs to either the hexagonal or the tetragonal system. Caution In some minerals, one n may be so close to another n that it may be falsely assumed that there are only two n s. Minerals of the tetragonal or hexagonal systems are uniaxial. 

Interference figures are useful in determining the orientation of a mineral particle and whether the mineral is uniaxial or biaxial, provided the particle is large enough to produce useful interference figures. A consideration of interference figures and interaxial angles is beyond the scope of this discussion. 

If n s of a mineral are determined, Tables can be consulted to determine the identity of the mineral. These are generally listed according to whether the mineral is uniaxial or biaxial, and whether the mineral is positive or negative (40). 

Hexagonal and tetragonal minerals (e.g, calcite CaC03, quartz SiO MgF2, tourmaline, BN) have one optic axis and are optically uniaxial. Orthorhombic, monoclinic, and triclinic minerals (e.g., sulfur, mica, turquoise, selenite) have two optic axes and are optically biaxial. 

For instance, a light beam traveling through calcite (CaC03), a uniaxial anisotropic mineral, is split into two rays that vibrate at right angles to each... 

The phenomenon of polymorphism was noted by Martin Heinrich Klaproth °i in 1798, when he proposed that the minerals calcite and aragonite must have the same chemical composition, CaCOa. Calcite forms a rhombohedral uniaxial crystal and is the stable form under normal conditions, with a density of 2.71 g/ml. Its metastable polymorph, aragonite, is an orthorhombic biaxial crystal with a density of 2.94 g/ml. This work was continued by Louis Jacques Thernard, Jean Baptiste Biot, and Eilhard Mitscherlich. Mitscherlich, for example, reported on it in his studies of phosphates and arsenates. The transition from calcite to aragonite has been studied at different pressures. ... 

The finely crystalline zeolitic matrix of tuffs cements the other non zeolitic particles and is responsible for the overall mechanical properties of the material. The compactness and the consequent physical-mechanical properties are somewhat variable as they are function of various parameters, such as mineral composition, tuff genesis, and original conditions of the deposit, especially as regards looseness of the parent material, grain size distribution, and others. Table 3 summarizes some data on unit weight, y, porosity, P, and uniaxial compressive strength, a, of a few tuff samples and other stone materials. 

Fillers are mainly used for reasons of economy, but in many cases they also improve some properties of the polymer. The most important fillers for polymers are minerals such as talc, chalk and china clay. Filler content generally used with plastics is up to 60 wt%. The most common practice is to feed the filler downstream into the melt by means of a twin-screw side feeder (Figure 6.3). It is well-known that thermoplastic melts with high loadings of small particles such as calcium carbonate, carbon black and titanium dioxide give both yield values in shear flow [58, 59], and uniaxial extension [60, 61]. 

Hydroxyapatite is, however, often present with mineral apatite, and it is also the principal inorganic component of bones and teeth. Its composition is similar to that of Dahlite (Table 2.6). About 85% of human body phosphorus is present as bone apatite (Chapter 11.1). Calcium (OH, F) apatite minerals are uniaxial negative they have a density of 2.9-3.2 g/cc and Moh s hardness of about 5. Fluorapatite is somewhat less soluble than hydroxyapatite, and below pH 4.8 the difference is greater. Fluorapatite minerals are usually soluble in HCl or HNO3. 

Column 4 lists the mineral optical properties either in transmisted hght with refractive index for isotropic n ), uniaxial e, co), or biaxial a, P>y, 2V), with the birefringence (<5),... 

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